Concrete is a low cost and the most industrially used material. This is primarily due to its high compressive strength and durability which until now is considered to be an indispensable building material. However, due to long term influence of surrounding environment, micro-cracking and damage of the concrete material are inevitable. Micro-cracks in particular are not easily detected due to limited technologies. Therefore, it is difficult to repair such undetectable cracks. When cracks are not restored timely and effectively, it leads to macro-crack formation that triggers brittle fracture and shortens concrete life, consequently threatens structure safety as well. Moreover, inspection and maintenance are difficult and expensive wherein the labor cost goes up to 50% of the repair cost. Thus, timely repair of micro-cracks in concrete is of major concern.
One of the most common methods of addressing this problem in concrete is by use of self-healing agents in microcapsules embedded within the concrete mix. Recently, examples of self-healing of concretes using microencapsulation techniques have been reported. For example, microencapsulated sodium silicate has been used as a healing agent. However, the microencapsulated sodium silicate has low silica (SiO2) content and is viscous. This results in less penetration into micro-cracks and thus inefficient healing of concrete. Other existing self-healing concretes using hollow fibers and hollow glass tubes are not feasible in commercial market. The use of concrete vibrator during construction process leads to the destruction of hollow fiber arrangement and design and the premature loss of the self-healing agent. The use of bacteria and calcium lactate inside clay particle as self-healing agent yields a very expensive aggregate (e.g. Euro 160/cubic meter) with an initial compressive strength 25% lower than normal concrete. In general, microcapsules produced to date are not cheap enough for mass production. Synthetic conditions and parameters are not environmental-friendly and are difficult to scale-up. There is also a problem of slow response between the microcapsules and the healing mechanism during micro-cracking.
There is a need to produce a self-healing material which is efficient in healing micro-cracks and suitable for mass production.